Electrode catheters have been in common use in medical practice for many years. Diagnosis and treatment of cardiac arrythmias by means of electrode catheters include mapping the electrical properties of heart tissue and selectively ablating cardiac tissue by application of energy. Such ablation can cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. Various energy delivery modalities have been disclosed for forming lesions, and include use of microwave, laser and more commonly, radiofrequency energies to create conduction blocks along the cardiac tissue wall.
In a two-step procedure—mapping followed by ablation—electrical activity at points within the heart is typically sensed and measured by advancing a catheter containing one or more electrical sensors (or electrodes) into the heart, and acquiring data at a multiplicity of points. These data are then utilized to select the tissue target areas at which ablation is to be performed.
In use, the electrode catheter is inserted into a major vein or artery, e.g., the femoral artery, and then guided into the chamber of the heart which is of concern. A reference electrode is provided, generally taped to the patient's skin. Radio frequency (RF) current is applied to the tip electrode, and flows through the surrounding media, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue, as compared to blood which has a higher conductivity than the tissue.
Heating of the tissue occurs due to its electrical resistivity. The tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue which is electrically non-conductive. During this process, heating of the electrode also occurs as a result of conduction from the heated tissue to the electrode itself. If the electrode temperature becomes sufficiently high, possibly above 60° C., a thin transparent coating of dehydrated blood can form on the surface of the electrode. If the temperature continues to rise, this dehydrated layer of blood can become progressively thicker resulting in blood coagulation on the electrode surface. Because dehydrated biological material has a higher electrical resistance than tissue, impedance to the flow of electrical energy into the tissue also increases. If the impedance increases sufficiently, an impedance rise occurs and the catheter must be removed from the body and the tip electrode cleaned.
In a typical application of RF current, circulating blood provides some cooling of the ablation electrode. Another method is to irrigate the ablation electrode, e.g., with physiologic saline at room temperature, to actively cool the ablation electrode instead of relying on the more passive physiological cooling provided by the blood. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions which tend to be larger and more spherical, usually measuring about 10 to 12 mm.
The clinical effectiveness of irrigating the ablation electrode is dependent upon the distribution of flow within the electrode structure and the rate of irrigation flow through the tip. Effectiveness is achieved by reducing the overall electrode temperature and eliminating hot spots in the ablation electrode which can initiate coagulum formation. More channels and higher flows are more effective in reducing overall temperature and temperature variations, i.e., hot spots. The coolant flow rate must be balanced against the amount of fluid that can be injected into the patient and the increased clinical load required to monitor and possibly refill the injection devices during a procedure. In addition to irrigation flow during ablation, a maintenance flow, typically a lower flow rate, is required throughout the procedure to prevent backflow of blood into the coolant passages. Thus, reducing coolant flow by utilizing it as efficiently as possible is a desirable design objective.
Another consideration is the ability to control the exact position and orientation of the catheter tip. This is ability is critical and largely determines the usefulness of the catheter. It is generally known to incorporate into electrophysiology catheters an electromagnetic (EM) tri-axis location/position sensor for determining the location of a catheter's distal end. An EM sensor in the catheter, typically near the catheter's distal end within the distal tip, gives rise to signals that are used to determine the position of the device relative to a frame of reference that is fixed either externally to the body or to the heart itself. The EM sensor may be active or passive and may operate by generating or receiving electrical, magnetic or ultrasonic energy fields or other suitable forms of energy known in the art.
U.S. Pat. No. 5,391,199, the entire disclosure of which is incorporated herein by reference, describes a position-responsive catheter comprising a miniature sensor coil contained in the catheter's distal end. The coil generates electrical signals in response to externally-applied magnetic fields, which are produced by field-generator coils placed outside the patient's body. The electrical signals are analyzed to determine three-dimensional coordinates of the coil.
U.S. Pat. No. 6,690,963, the entire disclosure of which is hereby incorporated by reference, is directed to a locating system for determining the location and orientation of an invasive medical instrument, for example a catheter or endoscope, relative to a reference frame, comprising: a plurality of field generators which generate known, distinguishable fields, preferably continuous AC magnetic fields, in response to drive signals; a plurality of sensors situated in the invasive medical instrument proximate the distal end thereof which generate sensor signals in response to said fields; and a signal processor which has an input for a plurality of signals corresponding to said drive signals and said sensor signals and which produces the three location coordinates and three orientation coordinates of a point on the invasive medical instrument.
Because of the size of the tip electrode and the limited interior space therein, the EM sensor is often positioned outside of the tip electrode, proximally thereof, and often off axis from the tip electrode which can reduce the accuracy of the position sensing capabilities of the sensor. Being outside the tip electrode, the position sensor is also exposed to bending stresses and can limit the flexibility and deflection of the distal tip section. Moreover, the sensor can be damaged by RF energy during ablation.
Where the distal tip is irrigated, the efficiency of irrigated cooling becomes a significant factor as ablation procedures can last five or six hours resulting in extensive fluid-loading in the patient. Conventional irrigated tip electrodes typically operate with a flow rate of about 17 ml/minute at below about 30 watts of RF ablation energy to about 30-50 ml/minute at about 30 watts or greater. The limited space in the distal tip may also lead to anchoring of the puller wires to a less desirable location such as a tubing wall causing tearing of the tubing wall and/or unintended asymmetrical deflection.
Accordingly, it is desirable that a catheter be adapted for mapping and ablation with improved cooling and position sensing characteristics by providing a tip configuration that includes housing in which the position sensor is protected and is located both distally and on-axis without inhibiting the flow and dispersion of irrigation fluid through the tip. It is also desirable that such a catheter exhibit symmetrical bi-directional deflection and that the walls of the catheter be damaged from deflection puller wires.